This invention relates to expansive cements. In another aspect, this invention relates to novel expansive compositions which have an improved ability to stress steel reinforcing members which are positioned within plastic concretes containing such compositions.
Portland cement concrete is one of the most widely used construction materials; however, it possesses the inherent characteristic of shrinkage on drying. This drying shrinkage can cause cracks in the resulting concrete. Cracks of varying size can generally be found in most reinforced concrete structures. In essence, when shrinkage of the curing concrete occurs, the concrete is placed under tension and if the tensile forces produced exceed the tensile strength of the material, a crack is produced.
As a consequence, expansive cements were developed so that concrete made from these cements would undergo a volumetric expansion during the curing cycle. The forces generated by this volumetric expansion of the concrete are harnessed in a manner that makes it possible to utilize these forces to control the cracking of the concrete caused by the drying shrinkage.
One technique for utilization of the volumetric expansion of concrete as a means of controlling cracking in the concrete is known as shrinkage compensation. This technique requires that the rate and amount of expansion in the concrete occur simultaneous with and in the same magnitude as the drying shrinkage. This balance of forces would eliminate the tensile stresses which normally give rise to cracking in concrete. In practice, such a match of expansion and drying shrinkage to achieve a nonshrinking or dimensionally stable concrete has been difficult to achieve and control as a great variety of materials and widely varying ambient conditions of temperature and relative humidity are encountered in concrete practice.
Another technique for utilization of volumetric expansion of concrete as a means of controlling cracking in concrete is to capture the force generated by the concrete expansion in reinforcing steel in such a manner that permits use of these forces to oppose the tensile stress generated by the drying shrinkage. This technique is referred to as stress induction. Reinforcing steel is placed in the fresh plastic concrete. During the setting reaction, the concrete forms a bond to or grabs the reinforcing steel. As the concrete expands, it carries the steel with it and thus creates a tensile stress in the steel. These tensile stresses in the reinforcing steel place the concrete under restraint and accordingly when expansion occurs, it is of a lesser magnitude and of a more controlled rate, and produces a stronger, denser concrete. This chemical stressing of the steel is analogous to mechanical prestressing of steel in concrete members in which a steel cable is placed and held in tension until the concrete has set and achieved a certain minimum strength. The cable is then released but within the concrete member it remains in a state of tension and places the concrete in compression.
In both cases, the tensile stresses within the steel reinforcement members exert compressive stresses on the concrete. The tensile stresses generated in concrete when drying shrinkage occurs are neutralized or balanced (because the concrete is in compression) until the magnitude of the drying shrinkage is greater than the magnitude of the tensile stresses of the steel reinforcement. It has been long assumed that as long as the measured expansion of the concrete exceeds any later drying shrinkage, the embedded steel reinforcement will maintain an overall state of compression within the concrete and no cracking will occur.
It has recently been found that all concrete expansion cannot be directly related to the induced tensile stresses within the reinforcing steel. It has been found that some slippage between the concrete and the steel occurs during expansion and therefore only a portion of the concrete expansion is utilized to stretch the steel.
For example, in a case where complete bonding occurs between the expansive concrete and the steel reinforcement bar, the measured expansion of the concrete is equal to the tensile strain on the steel. In this situation, the steel will keep the concrete in compression and for all amounts of drying shrinkage less than the maximum expansion prevent shrinkage cracking due to tensile forces set up by the decreasing volume. At the other extreme, if no bond occurs between the concrete and the steel, the steel bar remains slack and exerts no compressive stresses on the concrete. Subsequent drying shrinkage will place the concrete in tension and cause cracking. In this respect, the presence of the steel reinforcement contributes only to structural requirements and nothing to crack prevention.
In actual practice, expansive cement concrete will exhibit behaviors somewhere between these two extremes. Thus, there is needed an effective expansive cement whose degree of expansion can be easily controlled but yet will bond or grab steel reinforcement members more effectively such that a greater degree of concrete expansion can be directly related to the induced tensile stresses within the reinforcing member.
The earliest and most common type of expansive cements are the so called "Type K" compositions which are based upon a sulfoaluminate expansive mechanism. Examples of such compositions are disclosed in U.S. Pat. No. 3,251,701 and U.S. Pat. No. 3,303,037. The Type K expansive cements are produced by burning of a special clinker containing the proper amount of tetracalcium trialuminate sulfate (C.sub.4 A.sub.3 S). It is noted in cement nomenclature that C = CaO; S = SiO.sub.2 ; A = Al.sub.2 O.sub.3 ; F = Fe.sub.2 O.sub.3 ; and S = SO.sub.3. After hydration, this material generally forms ettringite and is accompanied by a concurrent increase in volume.
Another expansive cement composition based upon a sulfoaluminate expansion mechanism is the Type S expansive cement, which is a Portland cement containing a large amount of C.sub.3 A and modified by an excess of calcium sulfate above the usual amount found in Portland cement. Still another conventional expansive cement is the "Type M" cement which is either a mixture of Portland cement, calcium aluminate cement, and calcium sulfate, or an interground product made from the above respective cement clinkers.
The above expansive cement compositions have met with only a limited success due to the fact that the amount and rate of expansion have been difficult to control within acceptable parameters. Furthermore, even when such cements have exhibited controllable expansion, the poor ability to bond to or grab reinforcing steel has prevented the successful placement of expansive cement concrete.
Recently, expansive cements have been developed based upon calcium oxide reactions. Such expansive cements typically are formed by burning a clinker of argillaceous and calcareous materials to a degree such that the silicate is in the form of C.sub.3 S and substantially no C.sub.2 S exists in the composition, and an excess of free uncombined calcium oxide exists in the composition together with the other components in the form of a liquid phase containing C.sub.4 AF and C.sub.3 A. Compositions of this type are disclosed in U.S. Pat. No. 3,785,844 and copending patent application Ser. No. 404,934 filed Oct. 10, 1973, now U.S. Pat. No. 3,884,710. These expansive cements can be controlled to yield a wide range of expansion allowing them to be more effective than the sulfoaluminate cements for both shrinkage compensating and for stress inducing applications. However, even with this improved formulation, it was found that only a small percentage of the expansive potential was being used in concrete because of a partial bond to or grab of the reinforcing steel.